Method of mass spectral analysis with negative ions



W. H. BENNETT Oct;

' METHOD OF MASS SPECTRAL. ANALYSI$ WITH NEGATIVE IONS- 3 Sheets-Sheet 1Filed Aug. 8. 1951 INVENTOR ATTORNEYS INVENT OR 3 Sheets-Sheet 2 W. H.BENNETT METHOD OF- MASS SPECTRAL ANALYSIS WITH :NEGATIVE IONS Oct.23,1956

F i led Aug. 8, 1951.

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Application August 8, 1951, Serial No. 240,966

2 Claims. (Cl. 25.0-41.9)

This invention relates to methods of mass spectral analysis withnegative ions.

States Patent An object of the invention is to provide a method ofanalysis whereby it is possible to distinguish between gases of the samemolecular weight.

Other objects and further advantages will appear as this descriptionproceeds.

Practically all, if not all, prior art successful mass spectral analysishas been with positive ions but I have discovered ways of renderinganalysis with negative ions very useful in certain circumstances.

The invention is especially useful in cases where it is desired toanalyze a mixture having two or more gases of the same molecular weight.Very often in such cases there is a critical bombardment velocity atwhich one of the two gases will produce negative ions and the otherswill not. critical velocity and determining the number of negative ionsproduced, it is possible to conclude that a particular gas is present inthe mixture in a particular amount. For example, to take a very simplecase, it is impossible with prior art mass spectrometers to distinguishbetween carbon monoxide and molecular nitrogen because both havemolecular weights of 28. Hence, it has been impossible to detect smallpercentages of carbon monoxide in the presence of air. However, carbonmonoxide, if bombarded with 11 volt electrons, will yield negativeoxygen ions whereas nitrogen will not yield negative ions if bombardedwith electrons of any bombarding energy. Hence, the presence of smallpercentages of carbon monoxide may be determined by ap- Hence, bybombarding the mixture at this plying 11 volts accelerating potential tothe electrons used in the ion source and then determining the masses of.the negative ions produced. In that particular case, a complicatingfactor is that molecular oxygen also produces negative atomic ions, butsuch ions are produced .only with electron velocities in the vicinity of7 volts and none are produced with 11 volt electrons. Hence, bymaintaining the potential of the ion source at the critical potential of11 volts the desired determination may be made.

As another example of an application of my invention, assume that thereis amixture of three gases namely Cal-I4, CO and N2, all of mass 28. Bybombarding with 11 volt electrons producing negative oxygen ions it isthen possible to determine the quantity of CO in the mix ture bymeasuring the number of negative oxygen ions thereby produced. If thesame analysis were attempted by prior art methods one would secure massspectral lines for masses 1, 12, 13, 14, 15, 16, 24, 25, 26, 27 and 28resulting in confusion and error.

While I am not limited to any particular apparatus so Fatented Oct. 23,1956 an improvement on that disclosed in my published article entitledRadio Frequency Mass Spectrometer, Journal of Applied Physics, vol. 21,No. 2, pages 143 to 149, February 1950 and is also an improvement onthat disclosed in my prior copending application S. N. 196,024 filedNovember 16, 1950, entitled Radio Frequency Mass Spectrometer.

In the drawings:

Figure 1 is a part cross-sectional and part schematic diagram of oneform of tube that may be employed.

Figure 2 is a left hand end View of Figure 1.

Figure 3 is a right hand end view of the tube of Figure 1.

Figure 4 is a schematic diagram of a preferred hookup of the invention.

Figure 5 is a curve illustrating the yield of positive ions of anytypical gas for different ionizing potentials.

Figure 6 illustrates the negative ion yields of carbon monoxide fordifierent ionizing potentials.

Figure 7 illustrates the negative ion yield of oxygen for differentionizing potentials.

Referring to Figures 1 and 2, the tube employs a cathode 50 which hasleads 50a. A metallic cylindrical shield 51 has lead 51a for connectingthe shield 51 to a variable potential approximately the same as that ofthe cathode. Shield 51 has an aperture 51b which is about fourmillimeters in diameter. Cylindrical metal electrode 52 has an apertureat its lower end of about two millimeters in diameter and coaxial withaperture 5112. These parts are all mounted in an envelope 53 which hasan inlet opening 54 through which gases to be analyzed are admitted andit also has exhaust outlets 55 and 56 adapted to be connected to exhaustpumps for evacuating the tube. As is apparent, the exhaust outlet 55primarily exhausts the interior of cylindrical cup electrodes 51 and 52for the following purpose. The gas to be analyzed may have its chemicalcomposition somewhat changed if it strikes the very hot cathode.Therefore, in order to make sure that none of such gas with changedcomposition passes into the main analyzing chamber, the tube iscontinuously exhausted at outlet 55 whereby any gases of changedcomposition are promptly removed from the envelope 53. The gases to beanalyzed are continuously fed into inlet 54 at a slow constant rate andare continuously exhausted by exhaust pumps connected to outlets 55 and56.

The potential on accelerating electrode 52 is highly positive relativeto the cathode, say by thirtyvolts. As the electrons emerge throughaperture 52b they pass between the two parallel grids 61 and 63. Theaverage potential of these grids relative to the cathode will be termedP, for simplicity. Assume that it is desired to effect a value of P ofsay eleven volts, which as has been said is the desirable potential usedin detecting carbon monoxide. To carry out such an assumption the grid61 would be made ten volts positive relative to cathode 50 and the grid63 would be made twelve volts positive relative to the cathode 50. As aconsequence the electrons from cathods 50 form a narrow beam, known as apencil of electrons, as they emerge from aperture'52b at a potential ofthirty volts. They slow down to a speed of P volts as they enter therigion between grids 61 and 63. A magnet, or its equivalent in the formof suitable coils C, may be provided to produce a weak magnetic fieldwhich passes parallel to and in between the screens 61 and 63 andperpendicular to the electron stream emitted by the electron gun 50 52.In this case the magnetic field may have approximately thirty gauss. Themagnetic field could equally well be applied coaxial with the electronstream. In the latter case it may have a field intensity ofapproximately gauss or more. When the field is perpendicular to theelectron stream, as above stated, the potential diflerence between grids61 and 63 must be set at just a sufficient amount to give anelectrostatic field between grids 61 and 63 whose force on electronstraversing this space just equals the force in the opposite direction onthem due to the magnetic field. If P is the potential difference involts between the cathode 50 and the midpotential between grids 61 and63, then the potential difference (in volts) between grids '61 and 63should be equal to 0.0198 H.d. /P, where H equals the field gauss, and dequals the distance between grids 61 and 63.

in order that it can be determined how many electrons are beingproduced, an item which it is often desirable to know in calibrating andusing the tube, a small metallic electrode 62 is connected in serieswith a meter 1, a battcry B, and the cathode 50.

In Figure 1 and 2 coils M are shown, as they may be conveniently used inlieu of the magnet above referred to.

In adjusting the apparatus for use, the magnet, or coils C as the casemay be, is temporarily removed, the voltage on electrode 52 is adjustedto give maximum electron current, while the voltages on grids 61 and 63are both set at the potential P (say eleven volts) initially in order tofocus the beam. When the beam is in proper focus the magnet M isreplaced and the potentials on grids 61 and 63 are rendered slightlydifferent, according to the foregoing formula.

In view of the detailed disclosure in my published article, supra, andin my said prior copending application the theory of operation of thethree stage mass spectrometer herein referred to need not be stated indetail. Adjacent grid 63, there are grids 64 and 65 respectivelyconnected to leads 64a and 65a, and these grids are charged positivelyat potentials intermediate those of grids 63 and 66. The grid 64 tendsto draw the negative ions through grid 63 and grid 65 tends to furtheraccelerate the negative ions. Either or both of grids 64 and 65 may beomitted in which event the negative ions attracted into the mesh ofscreen 63 will then be attracted by grid 66.

There are three groups of grids of three grids each, the grids of eachgroup being closely spaced as compared to the spacing between groups.Grids 66, 66 and .66" are connected to lead 66a. Grids 67, 67 and 67"are connected to lead67a. Grids 68, 68 and 68 are connected to lead 63a.Leads 66:: and 68:: are connected to a source of positive polarity andlead 67a is fed with radio frequency potential. The distance betweengrids 67 and 67 is preferably different than that between grids 67 and.67 and the two distances preferably are according to the ratio of 7 to5.

The grids 69 may be used as repelling electrodes by applying a negativepotential thereto. This potential is selected as described in saidarticle and in said prior copending application, and is of value inrepelling background electrons and ions leaving only ions that attainedmore than a predetermined velocity. However, grids 69 may be omitted ifdesired. .Grid70 is connectedto cylindrical shield 70s and to lead 70awhich in turn may be connected to lead 69a. The collector 71 isconnectedto a source of positive potential through its lead 71a, and itwill attract negative ions that passed all of the other grids. Sincecollector 71 is made as positive as any other electrode in the tube, orpreferably more so, it will repel any positive ions that have passed thegrid 70.

Figure 3 illustrates in more detail, the leads from the tube. Theseveral gridsare knitted wire screens mounted in washer-draped discholders, which holders are supported by horizontal rods 72. Instead ofbringing the leads vertically throughthe side wall of the envelope theypass horizontally through the rods 72 and out the right hand end of theenvelope as shown in Figure 3. Cylindrical wire screens 73 connect grids66 and 66" and grids 68 and 6S inside of the tube. It should be notedthat the uppermost horizontal rod 72 is not directly between cathode 59and thespace betweengrids 61 and 63, hence there is nothing to preventelectrons from the cathode from passing to electrode 62.

In Figure 4 a complete schematic diagram for operating the tube as anegative ion spectrometer. The amplifier and measuring instrument 8!) isconnected between the collector 71 and ground, and measures the fiow ofnegative ions to the collector 71. Variable frequency oscillator 81provides the variable radio frequency potentials, and for many types ofwork a suitable frequency range may be from kilocycles to 10 megacycles.The output of oscillator 81 is fed through condenser 33 and reactor 84.The drop across reactor 84 is fed to the middle grids 67, .67 and 67",and also the leads 66a and 66a through resistors 66b and 68b. Vacuumtube voltmeter 82 measures the potential of the oscillator 81 wherebythe latter may be adjusted and held constant.

in order to fully understand the theory of this invention it isdesirable to review some basic facts. I11 Figure 5 there is illustratedthe yield of positive ions for a typical gas as the ionizing potential Pis increased. This curve assumes that a typical gas is admitted in to atypical mass spectrometer tube. The ionizing potential P, in theapparatus shown in Figure l, is the average potential of grids 6163,relative to the cathode.

Figure 6 shows that in the case of carbon monoxide there are alsonegative ions produced at an appearance potential in a narrow range often to twelve volts and again in a broad range at voltages in excess oftwenty volts.

Figure 7 shows the yield of negative ions when oxygen is the only gas inthe tube. As shown there is no yield of negative ions for potentialsbetween about eight and sixteen volts.

Nitrogen gas and inert gases will not yield negative ions when bombardedwith electrons.

'With the foregoing background information assume a mixture of gasesthat may contain nitrogen C2H4 and carbon monoxide. Further assume thatit is desired to know if carbon monoxide is present, and if so, howmuch. First an analysis, may be made according to ordinary methods ofmass spectrometry as contemplated in said article and in my said priorcopending applications. This would give indications for mass linesnumbers 1, 12, 13, 14, 15, 16, 24, 25, 26 27 and 28. It could beconcluded that there was present a mass of value 28 but it would beimpossible to say what it was or how much of it was present.

According to the present invention the unknown mixture is fed into inlet54 after the focus is suitably adjusted and the potential P set forabout ten to twelve volts. If the bombardment produced negative ions formass line 16 it would be obvious that carbon monoxide was present. Byadjusting P to give the maximum intensity and measuring the number ofsuch ions it is possible to conclude how much carbon monoxide is in themixture. This would be done with the connections shown in Figure 4.

-While I have illustrated my invention by showing how to detect carbonmonoxide in the presence of nitrogen and ethylene, it is understood thatthe broader aspects of this invention are not limited to particulargases but cover the steps whenever applied to gases involving similarproblems to those discussed herein.

To illustrate how the present method may be performed with otherapparatus, I will describe now how the method may be practiced with theapparatus shown in Figures 1 and 2 of U. 5. Patent No. 2,387,786 to H.W. Washburn, granted October 30, 1945, entitled Analytical System. Topractice my method, a change in the Washburn apparatus is required inthat the polarity of electrodes 21 and 23 ,with respectto each othermust be reversed. This may be done by reversing the polarity of thebattery in the ion beam deflection control circuit. The electrode 25will then be highly positive. With this modified apparatus one wouldpractice my method by fixing the potentialbetween the cathode ,17 andthe drift space between electrodes 21 and 23 at the critical value,which in the case of carbon monoxide is ten to twelve volts. A verysensitive recorder would be employed and would indicate the presence ofnegative oxygen ions and thus indicate the presence of carbon monoxide.

I claim to have invented:

1. The method of analyzing a gaseous mixture for the presence of aparticular component thereof comprising the steps of bombarding thegaseous mixture with elec trons of lowest energy range known to producenegative ions from the particular component, providing a segregatingfield of intensity required to segregate the negative ions from saidparticular component from ions of said other components of said gaseousmixture, and indicating the amount of said segregated ions present.

2. The method of analyzing a gaseous mixture for the presence of aparticular component thereof, said component being characterized in thatit will produce negative ions when bombarded with electrons withineither of two ranges of velocities one of which ranges is a narrow rangeof velocities as compared to the other and the ionizing electronvelocities of the narrow range are lower than those of the wider range,comprising the steps of bombarding the gaseous mixture with electronswhose velocities are within said narrow range, producing a segregatingfield of intensity required to segregate the negative ions from saidparticular component from ions of other components of said gaseousmixture, and indicating the presence of said segregated ions.

References Cited in the file of this patent UNITED STATES PATENTS2,221,467 Bleakney Nov. 12, 1940 2,490,278 Nier Dec. 6, 1949 2,535,032Bennett Dec. 26, 1950 OTHER REFERENCES Radio Frequency MassSpectrometer, National Bureau of Standards Tech. News Bulletin, vol. 32,September 1948, pages -108.

Radio Frequency Mass Spectrometer, by Bennett, published in Journal ofApplied Physics, vol. 21, February 1950, pages 143-149.

A Mass Spectrum Analysis of the Products of Ionization by ElectronImpact in Nitrogen Acetylene, Nitric Oxide Cynagen and Carbon Monoxide,by Tate et al., published in Physical Review, vol 48, September 15,1935, pages 525-531.

